1. Field of the Invention
The present invention relates to a communication device that is configured to perform transmission and reception in a plurality of frequency bands, and particularly relates to a communication device including a duplexer.
2. Description of the Related Art
To date, a variety of communication devices that use a plurality of frequency bands have been proposed. For example, Japanese Unexamined Patent Application Publication No. 2004-266361 discloses a communication device that can process a transmission signal and a reception signal of a first frequency band, a transmission signal and a reception signal of a second frequency band, and a transmission signal and a reception signal in a third frequency band. In this case, a duplexer that processes the transmission and reception signals of the first frequency band, a triplexer that processes the transmission and reception signals of the second frequency band, and a duplexer that processes the transmission and reception signals of the third frequency band are connected to an antenna terminal.
The duplexers and triplexer are configured using a SAW filter in order to reduce size and increase selectivity. Furthermore, in the communication device, it is possible to perform transmission and reception at the same time in the first to third frequency bands.
In the communication device described in Japanese Unexamined Patent Application Publication No. 2004-266361, if transmission was performed at the same time with a transmission signal of the second frequency band, noise was liable to occur in the first frequency band and the second frequency band. This is because, if a transmission signal of the first frequency band enters the triplexer that handles transmission and reception in the second frequency band, the first transmission signal and the second transmission signal cause intermodulation.
In other words, with a conventional communication device that handles signals of a plurality of frequency bands, if a plurality of transmission signals are transmitted at the same time, a transmission signal transmitted by a duplexer or a triplexer may enter another duplexer or triplexer, and the plurality of transmission signals may cause intermodulation.
It should be noted that a configuration is known in which a band elimination filter that uses a surface acoustic wave element is connected between an antenna terminal and a multiplexer in order to suppress noise due to the intermodulation distortion. In other words, noise caused by intermodulation distortion is suppressed by using a surface acoustic wave element and connecting a band elimination filter that eliminates signals of the frequency bands processed by other multiplexers. However, even in this kind of configuration, it should be noted that noise caused by intermodulation distortion due to the plurality of transmission signals is still liable to remain.